Generally, a fuel cell system, having a fuel cell stack, a oxidant flow field, a fuel gas flow field, and a coolant flow field, generates electric energy utilizing an electrochemical reaction between a fuel gas and an oxidant. Such a fuel cell system is generally provided with a Membrane Electrode Assembly (MEA).
The MEA has a membrane electrolyte sandwiched between an anode supplied with humidified fuel gas and a cathode supplied with humidified oxidant. The MEA is again sandwiched between a pair of separators, where flow fields, which are defined as a plurality of channels or grooves, are formed for flowing of the fuel gas and oxidant.
Furthermore, the fuel cell system is provided with a coolant flow field for cooling the fuel cell stack. Each of the oxidant flow field, the fuel gas flow field, and the coolant flow field is provided with a pump, a blower, and a plurality of valves and so on.
When a fuel cell as described above is adapted in a fuel cell vehicle, the fuel cell is repeatedly turned on and off, dependant on whether the vehicle is driven or not. When the fuel cell operates, water generated by electrochemical reaction of the fuel cell is drained, but when the fuel cell stops operating, water failing to be drained remains in the fuel cell stack. Furthermore, when the fuel cell stops operating after driving, the humidified fuel gas and the humidified oxidant in the flow fields also remain in the plurality of grooves or channels. When the atmospheric temperature drops to a low temperature, water of the residue can freeze, deteriorating the MEA of the fuel cell and blocking the flow fields, and the parts provided with each of the flow fields. Accordingly, in order to guarantee the safety of the fuel cell, the residue in the fuel cell system must be removed after the fuel cell is turned off.
Conventionally, in order to solve the problem described above, the residue in the fuel cell is removed by nitrogen purging. However, when nitrogen purging is used, additional apparatuses such as a storage tank for nitrogen and a controller for adjusting the nitrogen pressure are required, and the storage tank must be repeatedly recharged. Furthermore, in order to completely remove the residue, a substantial amount of time and high consumption of nitrogen are required.